This patent relates to capacitive transducers, and more particularly to techniques for overcoming electromagnetic interference in capacitive sensors.
In inertial sensors, electromagnetic disturbance or interference (EMI) occurs primarily due to capacitive coupling between bond wires and nearby cables, plates, circuitry, etc. FIG. 1 illustrates an exemplary scenario of EMI. In FIG. 1, a microelectromechanical structure (MEMS) device 102 is coupled to an application specific integrated circuit (ASIC) 104 by a plurality of bond wires 106. A source of EMI 110 that is near the bond wires 106 creates capacitive coupling 110 between the EMI source 110 and the bond wires 106. Capacitor symbols are shown in FIG. 1 to illustrate the capacitive coupling 110, but this simply illustrates the parasitic capacitance between the electromagnetic disturbance source 110 and the bond wires 106, no actual electrical component is present. The bond wires coupling capacitive nodes are the most sensitive to EMI, as opposed to nodes driven by a voltage source or amplifier.
In environments with a high density of electronics, there can be numerous sources of EMI, and these EMI sources can be significant. The electromagnetic disturbances can also occur at substantially a single frequency, which upon sampling can get folded into a DC component. These electromagnetic disturbances can land on top of a desired sensor signal and obliterate the desired signal. For example, if a desired signal is sampled at a 100 kHz clock frequency, and the disturbance is at 100 kHz, then when sampling the disturbance at the clock frequency it can appear as a substantially DC signal. Thus, it is important to protect desired sensor signals, especially along capacitive paths, from EMI. The EMI problem is especially important to solve in safety critical applications that are in harsh environments, for example the sensors used for electronic stability in an automobile.
The two commonly used solutions to EMI are shielding the sensor with metal, and using a differential approach. Shielding the sensor with metal includes creating a Faraday cage to block external electric fields which can cause EMI. However, shielding can be bulky and expensive, especially when there are numerous sensors to be shielded or there is a high density of electronics to be fit in a small area.
The differential approach takes the differences between signals on parallel wires which can substantially subtract out the electromagnetic disturbance as a common mode signal. FIG. 2 illustrates the differential approach. The exemplary differential sensor and amplifier system 200 includes a MEMS device 220 coupled to an ASIC 240 by bond wires 260, 262. Each of the bond wires 260, 262 experiences EMI from external EMI sources 210. There is capacitive coupling 250 between the EMI source 210 and the first bond wire 260 creating a first disturbance capacitance C1, and there is capacitive coupling 252 between the EMI source 210 and the second bond wire 262 creating a second disturbance capacitance C2. If the disturbance capacitances C1 and C2 between the EMI sources 210 and the bond wires 260, 262 are the same, then the electromagnetic disturbance is rejected due to the common mode rejection of the differential amplifier of the ASIC 240. However, in order to achieve the desired cancellation, the disturbance capacitances C1 and C2 between the EMI sources 210 and the bond wires 260, 262 should not be mismatched by more than 0.5%. This matching can be very difficult to achieve in practice. Even if the matching is achieved initially, bond wires can be disturbed or warped, for example by an automobile accident. This movement of the bond wires can cause asymmetry between the bond wires, which can cause an unwanted mismatch in the disturbance capacitances and reduce the effectiveness of the differential approach.
It would be desirable to have a robust technique for reducing electromagnetic interference that also overcomes some of the disadvantages of shielding and differential circuits.